Prefilled syringes, each of which is preliminarily filled with a medicinal liquid, have been known. Such prefilled syringes each include an outer tube provided with a discharge port at its distal end, a gasket inserted into the outer tube, and a plunger connected to the gasket. The medicinal liquid is enclosed in a space surrounded by the outer tube and the gasket.
In the case of injecting a medicinal liquid, for example, into a living body by use of the prefilled syringe, a needle assembly having a hollow needle is mounted on the discharge port of the outer tube, whereby the prefilled syringe is used in such a mounted condition (see, for example, Patent Document 1). Further, in the mounted condition, the inside of the outer tube communicates with the inside of the hollow needle. The needle assembly (protective sleeve) described in Patent Document 1 includes a hollow needle, a tubular outside member (outside sleeve member) disposed on an outer peripheral side of the hollow needle and supporting and fixing a base end portion of the hollow needle, a tubular inside member (inside sleeve member) disposed between the outside member and the hollow needle, the tubular inside member being coaxial with the outside member and movable along the axial direction of the outside member, and a coil spring for biasing the inside member toward a distal end side (in the distal direction). In the needle assembly having such a configuration, by being moved relative to the outside member in the axial direction thereof, the inside member can assume a first state in which a needle body inclusive of a needle point is covered, and a second state in which the needle point is exposed. In addition, under a biasing force of the coil spring, the inside member can be maintained in the first state.
In the needle assembly described in Patent Document 1, however, if only the inside member in the first state is pushed toward the base end side (in the proximal direction) against the biasing force of the coil spring, the inside member is capable of being brought into the second state, even after the needle assembly has been used once. Therefore, in the case that such a pushing force is unintentionally exerted on the inside member, the inside member is brought into the second state, leading to the possibility that a finger or the like may mistakenly be pierced by the needle point, which is exposed from the inside member.
Patent Document 1: Japanese Patent No. 2872318